In the middle of an industrial estate in Zaandam, just north of Amsterdam, stands the newest prison in the Netherlands.

The prison is built by the side of an old wood yard

But the word "stands" is not quite right, because this prison is in fact moored on one of the country's many waterways.

And the inmates in this floating prison are not criminals but illegal immigrants, guilty of what the Dutch call an "administrative offence".

This is the answer to a problem the authorities faced in the late 1990s - how to separate illegal immigrants from ordinary criminals when you already have overcrowded prisons.

"It's easier to get a place on the water than to find land, plus it's easy to build," says Erik Nijman from the Dutch ministry of justice.

And he says a floating prison is also more flexible: "If we have a problem for example in Amsterdam, we can transport them over water."

'No easy option'

Built by the side of an old wood yard, the prison sits on two concrete platforms, each in turn moored to large steel pilings.

Each cell holds two people and has TV, fridge and a coffee machine

On dry land next door are two white domes specially designed by a Dutch artist. Made of a lattice-work of metal, this is where the inmates play sports like football or basketball.

To enter the prison, you walk across a small bridge. Inside the corridors of cells are bright and clean. There are communal areas with table football, table-tennis and payphones.

The cells themselves hold two people, with bunk beds, a desk, fridge, TV, even a coffee machine. An en-suite bathroom is fitted with a toilet and shower.

When the prison is full it will house 576 illegal immigrants.

But despite state-of-the-art facilities, this is no easy option.

Cells are locked for 15 hours a day, from 1700 to 0800. Inmates can stay for six months or longer, as the authorities try to identify them and persuade them to go home voluntarily.

Nine-months deadline

The International Organisation for Migration (IOM) plays a crucial role here. They try to persuade people to travel home, providing help with tickets, passports and even cash.

The prison can house nearly 600 people

And although they do not like the idea of locking up illegal immigrants, they do support separating them from criminals.

"It's a concern to society if people with an administrative offence are put behind bars together with criminals who have committed a real crime," says Joost van der Aalst, the IOM head in the Netherlands.

"So, to have a mixed population isn't a good idea, and that's one of the reasons why the Dutch government has decided to establish floating detention centres for aliens."

But Mr van der Aalst admits only about 20% of those contacted by the IOM agree to return voluntarily.

If the authorities cannot identify someone properly, Dutch law says that after nine months in detention they have to be released back onto the streets.

Living on the margins

Dutch police do not actively seek out illegal immigrants, but if someone is questioned by the police and does not have the right papers, they will be detained.

Galina has now decided to go back to Ukraine

As a result people like Galina, a Ukrainian university graduate, live on the margins of society.

She was smuggled into the Netherlands from Poland after paying a local man her entire savings of 1,100 euros (£786). But promises of work soon evaporated and she has struggled to get by.

"It's very difficult because when I go across the street and see the police, every time I am afraid I will go to prison. But I only want to work and live here legally. It's a problem."

After five hard months, Galina has decided to head for home. And the Dutch authorities are hoping that a stay in their new floating prison will persuade other illegal migrants to follow her example.